www.nature.com/scientificreports OPEN Computer simulation study of the penetration of pulsed 30, 60 and 90 GHz radiation into the human ear Zoltan Vilagosh1,2*, Alireza Lajevardipour 1,2 & Andrew Wood1,2 There is increasing interest in applications which use the 30 to 90 GHz frequency range, including automotive radar, 5 G cellular networks and wireless local area links. This study investigated pulsed 30–90 GHz radiation penetration into the human ear canal and tympanic membrane using computational phantoms. Modelling involved 100 ps and 20 ps pulsed excitation at three angles: direct (orthogonal), 30° anterior, and 45° superior to the ear canal. The incident power fux density (PD) estimation was normalised to the International Commission on Non-Ionizing Radiation Protection (1998) standard for general population exposure of 10 Wm−2 and occupational exposure of 50 Wm−2. The PD, specifc absorption rate (SAR) and temperature rise within the tympanic membrane was highly dependent on the incident angle of the radiation and frequency. Using a 30 GHz pulse directed orthogonally into the ear canal, the PD in the tympanic membrane was 0.2% of the original maximal signal intensity. The corresponding PD at 90 GHz was 13.8%. A temperature rise of 0.032° C (+20%, −50%) was noted within the tympanic membrane using the equivalent of an occupational standard exposure at 90 GHz. The central area of the tympanic membrane is exposed in a preferential way and local efects on small regions cannot be excluded. The authors strongly advocate further research into the efects of radiation above 60 GHz on the structures of the ear to assist the process of setting standards. Tere is increasing interest in wireless communication systems such as the 5 G mobile networks using the 30 to 90 GHz frequency band1. Devices operating in this band use less power per unit of data transmitted than the current frequencies, and the band itself ofers data rates in the order of 10 Gigabits per second. Additional appli- cations in the neighbourhood of 30 to 90 GHz include Wireless Local Area Networks (WLAN) operating at 60 GHz2,3, millimetre wave radiation heating for material processing4, automotive radar at 24–29 GHz and 76–81 GHz5, and the Active Denial anti-personnel system at 94 GHz6. Te deployment of these applications will inevi- tably have an impact on the environmental exposure of humans. Te absorption coefcient of liquid water increases from about 3500 m−1 (35 cm−1) at 30 GHz to 7500 m−1 (75 cm−1) at 90 GHz7. It follows that the presence of water in the atmosphere produces signifcant signal loss, which necessitates denser communication networks with transmitters placed at many angles and elevations. Te water content of living sof tissues is in the order of 70–75%, thus most of the energy from any incident radia- tion is strongly absorbed in the frst few millimetres of sof tissue. Te study of 30–90 GHz radiation exposure inevitably becomes focused on the skin, the cornea of the eye and the ear canal. Te International Commission on Non-Ionizing Radiation Protection (ICNIRP,1998)8 standard for exposure to the 10–300 GHz frequency radi- ation is stated in terms of incident power fux density (PD). Te existing PD exposure limits for 10–300 GHz radiation are given as 10 Wm−2 for the general public and 50 Wm−2 for occupational contact. Te PD values translate to an incident orthogonal electric feld intensity of 61.4 Vm−1 and 137 Vm−1 respectively, in air. Te draf ICNIRP (2018)9 recommendations increase the maximum PD to 20 Wm−2 and 100 Wm−2 for the general public and occupational exposure respectively. Te revised standard is to be released shortly, but the results of this study are easily scalable to any new exposure limits. Given that the human tympanic membrane (ear drum) is exposed to the outside environment through the outer ear canal, there is a need to adequately explore the penetration of 30–90 GHz radiation into the ear. Tere are no specifc ear exposure guidelines in the ICNIRP (1998) standards. Te anatomy of the human ear shows large individual variability10,11. Te external ear canal is approximately cylindrical. Te radius of the adult human canal is 3.0 to 4.5 mm, larger in the vertical dimension than the 1Swinburne University of Technology Melbourne, Melbourne, Australia. 2Australian Centre for Electromagnetic Bioefects Research, Melbourne, Australia. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:1479 | https://doi.org/10.1038/s41598-020-58091-7 1 www.nature.com/scientificreports/ www.nature.com/scientificreports horizontal and sloping upward and forward from its external opening. Te canal becomes wider inferiorly at the tympanic membrane. The waveguide cut-off frequency for electromagnetic radiation for a cylinder is expressed as f(cutoff) = 1.841 C/2πa, where f(cutof) is the cutof frequency below which the cylinder will not function as a waveguide, C is the speed of light in the particular medium and a is the radius of the cylinder. Given that the radius the ear canal is 3.0–4.5 mm, the expected waveguide cutof is 18–30 GHz. It follows that the radius of the ear canal does not impede the propagation of the radiation at 30–90 GHz. Te absorption and refection properties of the tissues of the outer ear, the ear canal and the tympanic membrane become important considerations. Te difraction from the structures of the outer ear and the canal entrance is also a signifcant factor as it modifes or impedes the progress of the radiation. Signifcant penetration of the radiation into the ear canal at 60 GHz has been demon- strated12 and simulations at 300 GHz have shown that 54% of the PD presented to the front of the ear penetrates into the tissues of the tympanic membrane13. Tere are no comparable comprehensive exposure studies for the 30–90 GHz range. Te lining of the outer region of the ear canal resembles normal thin skin. Te deep part of the ear canal, on the other hand, has no dermis, a thin 0.01–0.02 mm layer of epidermis and a thicker 0.10 mm layer of stratum cor- neum (SC) compared with normal skin as the area is not abraded mechanically14. In adult humans, the tympanic membrane is approximately 25 mm from the ear canal entrance. Te tympanic membrane slopes inward from its superior margin at an angle of 25–30°. Te thickness of the tympanic membrane varies with age. Te adult tympanic membrane has four layers; two epidermal layers of about 0.02 mm thickness on the outer aspect (con- sisting of a cornifed SC layer and a living basal layer), a fbrous tissue layer of 0.02–0.230 mm (thicker superiorly in the region of the pars faccida and thinner in the pars tensa), and a 0.02–0.03 mm mucosal lining on the inner aspect of the tympanic membrane9. Tere is a system of epithelial cell migration which is unique to the tympanic membrane and the ear canal which serves to move cells towards the canal entrance15, continually clearing debris from the canal. Te tympanic membrane is well supplied with blood, richly innervated and highly sensitive to infammation and mechanical insult. Tympanic membrane damage may lead to pain or hearing loss. Small changes in the local environment may elicit noticeable symptoms. Heat transfer from the blood fow within the tympanic membrane is supplemented by the difusion of heat generated by metabolic processes within the brain and the proximity of the carotid artery and jugular vein16 and heat loss is aided by loss of heat to the air in the ear canal. Te tympanic membrane is thin and is suspended between two air containing cavities of the middle ear and the ear canal, and thus capable of rapid radiative heat loss. Te tympanic membrane may be infamed, increasing the blood fow and temperature or have scarring from past perforations or grommet tube insertions with a reduced blood fow in the scarred area. Tese features make any thermal efect of a given dose of direct radiation on the tympanic membrane difcult to evaluate. Estimates of temperature rise are, however, possible using the known mass den- sity and thermal properties of the constituent tissues. Te function of the tympanic membrane is determined by its structure rather than any specialised tissue components. Any non-thermal efects are likely to be comparable to similar tissues at other sites and need not be studied directly, however, the structures beyond the membrane, such as the vestibular apparatus and cochlea, do contain highly specialised and sensitive tissues, and the level of radiation penetrating to these areas needs particular evaluation. Te study of the human ear is hampered by a lack of a suitable animal model. Te usual laboratory animals, such as mice, rats, guinea pigs, rabbits and pigs, have either a disproportionate tortuosity of their ear canal, or have obliquely angled or small tympanic membranes17. Given these difculties, the use of a computational phan- tom modelling becomes useful for performing the preliminary studies. Te aim of the simulations was to investi- gate the proportion of the radiation that would reach the tympanic membrane at 30, 60, and 90 GHz frequencies in the frst instance, and then study the PD, the specifc absorption rate (SAR), the temperature rise within the membrane itself and the subsequent propagation of the radiation into the middle ear. Results The attenuation and distribution of the pulsed signal within the ear canal. Te difraction at the outer ear entrance and the refections from the outer ear structures and within the ear canal change the Gaussian pulses (see Methods) into more complex forms.